Nucleic acids perform two important functions
1. Replication: It is the property of biomolecules to synthesise exact copy of it. DNA has this unique property of duplicating itself. The genetic information for the cell is coded in the form of the sequence of bases adenine (A), thymine (T), guanine (G) and cytosine (C) in the DNA molecules. During division of cell, DNA molecules replicate and produce exact copies of them. Each daughter cell has DNA molecules identical to that of the parent cell. According to a prominent theory of DNA replication, the two strands of DNA helix unwind and each strand serves as a template of pattern for the synthesis of a new strand in the cell environment. The specificity of base pairing ensures the exact duplication of sequence of bases in the reproduced strand of (DNA).
2. Protein synthesis: The DNA molecules in the cell nucleus hold the code for the protein synthesis. The genetic information is coded in DNA in the form of specific sequence of bases. The synthesis of a specific protein involves the following two steps:
Transcription involves copying of sequence of bases from the DNA strand on the RNA molecule called messenger RNA (mRNA). The bases of the mRNA are complementary to those of the DNA strand. Messenger RNA contains only four bases adenine (A), guanine (G), cytosine (C) and uracil (U). DNA molecules contain the four adenine (A), guanine (G), cytosine (C) and thymine (T). the complementary base pairs are as follows:
| DNA |
mRNA |
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T) |
Uracil (U)
Cytosine (C)
Guanine (G)
Adenine (A) |
After transcription mRNA moves from the nucleus of the cell to a ribosome in the cytoplasm where it serves as template for the protein synthesis.
The sequence of bases in mRNA is read in a serial order in groups of three at a time. Each triplet of nucleotides (having a specific sequence of bases) is known as codon. Each codon specifies one amino acid. Many amino acids have more than one codons. The amino acids are brought to the mRNA by another type of RNA and called transfer RNA (tRNA). Each amino acid has at least one corresponding tRNA. At one end of the tRNA molecule is a trinucleotide base sequence that is complementary to some trinucleotide base sequence on mRNA (anticodon). The other end of tRNA molecule on mRNA is (anticodon). The other end of tRNA molecule carries a specific base sequence of three nucleotides carries a specific base sequence of three nucleotides - CCA - with a - OH group on the sugar exposed on the terminal adenine nucleotide (amino acid attachment site).
This - OH group combines with a specific amino acid and carries it to the mRNA. The complex between mRNA and tRNA is stabilized by another type of RNA called ribosome RNA (rRNA). After transfer of amino acid, the tRNA is free to go back and repeat the process. Thus, proteins with a specific sequence of amino acids are produced. The sequence of amino acids in proteins is determined by the sequence of nucleotide bases on mRNA which in turn is determined by the sequence of bases in DNA molecule. The DNA sequence that acts as a code for a specific protein is called gene. Every protein in the cell has a corresponding gene. The relationship between the nucleotide triplets and the amino acids is called genetic code.
The genetic code has four noteworthy features:
(i) It is universal.
(ii) It is degenerate. It means that more than one codons can act as a code for an amino acid.
(iii) It is commaless.
(iv) The third base of the codon is less specific. The first two bases of the codon are most important.
Synthesis of proteins in the body takes place at very fast rate. Only minutes after the radioactive amino acids are injected into the animals, radioactive proteins can be found.
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